How to write a book for all ages.

Well, if you're a publisher or an agent, I'm sure the thought has just crossed your mind, something like this, "Oh, no, every novel must have a category. How else would we submit or find a place on the shelves to stock it." Really? BS, if I've ever heard it. Yes, there are plenty of novels that are strictly adult fiction and bunches that appeal only to youth, but there really is an opened ended category I call--to make it simple--"All ages." This is how I think you get there.
Several ways come to mind, but I think the simplest is to have the story told in the first person by or about a young person--8 to 18. The Harry Potter series comes to mind. The story isn't told in the first person, of course, but it's all about Harry. Basically, Harry is a young boy caught up in a series of adult situations, and as we all know, the Harry Potter readers are from 8 to 88. To Kill a Mocking Bird also comes to mind as well as A Painted House. I think the key to satisfying your readers is to give them a taste of youth combined with an intertwining set of relationships with adults. You can build on this relationship with your readers by bringing in a nostalgic element which may remind them of their childhood. However, your basic task boils down to your skill with building a story line that will intrigue all of your readers.
In my novel, The Red Scarf, an 11 year old paper boy tells the story of his funny, sad, and sometimes exciting times between Thanksgiving and Christmas. It's set in 1944. An attorney friend of mine took the book to New York on a business trip. I quote, "My wife had to punch me during the flight to New York, because I was laughing out loud. But on the way back as I finished the book, I had tears."
I think there are several way to sum up the "all readers" status of The Red Scarf. First, the setting: Poor, country boy struggling to earn money---the nostalgic element--set in 1944. The adults that interact with the two boy--Peg, the pool hall owner, Wing, the city marshal, Doc, the newsstand owner, Big Six, the ornery roughneck, and Uncle Hugh, the old black man. "Uncle Hugh is one of our bestest friends," said John Clayton.
Did it make the all ages cut? Well, take a look at the review from Amazon.com I've included in my post.

A. Bryant "Arkansas Bred" (Little Rock, AR USA) - See all my reviews
This review is from: The Red Scarf (Hardcover) Every single person I have shared your book with has loved it. My father just finished it and absolutely could not say enough wonderful things about it. In fact, he asked me if the New York Times had read it and said they really needed to share it with the rest of the world. And, he's buying a copy to give to my Uncle for Christmas. My 9-year-old son even told me how much some of the kids at his school were enjoying reading it and grabbed his copy to start reading it on his own. That is really, really saying something when a piece of literature can cut across so many generations and bring so much Christmas joy to so many. Congratulations and thanks for sharing a piece of your Arkansas childhood with the rest of us!